The Kelvin method for the measurement of work function can be employed for the analysis of a wider range of materials, at different temperatures and pressures, than any other surface analysis technique. Work function is a very sensitive parameter which can reflect imperceptible structural variations, surface modification, contamination or surface-related processes. The method is now regaining popularity1-4 as a powerful technique because of its inherent high surface sensitivity, high lateral resolution due to the availability of nanometric precision-positioning systems, and improved signal detection devices. Unlike many other methods, the measurement of work function does not depend on an estimate of the electron reflection coefficient on the surface. Moreover, the technique does not use high temperature, high electric fields, or beams of electrons or photons. Being a non-contact and non-destructive method, it does not pose the risk of desorbing or removing even weakly-bound species from the surface. Furthermore, the Kelvin method is a direct measurement method requiring only a simple experimental set-up with no sample preparation.
When an electron is removed from a point within a material, the total change of thermodynamic free energy of the whole system is the difference between the change of the electrochemical potential of that material and the change of the electrostatic potential of the electron. If the electron is removed from a surface to a point in a vacuum, far from the outside surface so the surface forces have no more influence on the electron, this change of free energy is called the work function of that surface. The corresponding change when the electron is removed to another material that is in intimate electrical contact and thermal equilibrium with the first material, is called the contact potential difference (CPD). For example, when two different conductors are first brought into electrical contact, free electrons flow out of the one with the higher electrochemical potential (i.e., Fermi level) into the other conductor. This net flow of electrons continues until equilibrium is reached when their electrochemical potentials have become equal. The metal of higher work function (having originally a lower electrochemical potential) acquires a negative charge, the other conductor being left with a positive charge. When the whole system reaches thermodynamic equilibrium, the resulting potential difference is the CPD and is equal to the difference between their work functions.
In order to measure the CPD it is necessary to connect the conductors. A direct measurement with a voltmeter included in the circuit is not possible, since the algebraic sum of all the CPDs in the circuit is zero. Thus, CPD must be measured in an open circuit i.e., using a dielectric such as a vacuum or air between the conductors.
The Kelvin method is based on a parallel plate capacitor model: a vibrating electrode suspended above and “parallel” to a stationary electrode. The sinusoidal vibration changes the capacity between plates, which in turn, gives a variation of charge generating a displacement current, the Kelvin current, proportional to the existing CPD between the electrodes.
The last century witnessed a continuous process of improving and modification of the Kelvin probe in order to adapt it for particular applications5-10. The probe has been used in surface chemistry investigations, surface photo voltage studies, corrosion, stress, adsorption and contamination studies and was adapted for measurements in liquids, at high temperatures, in ion or electron emitting samples or in an ultra high vacuum environment11-15. The problem of conducting measurements at the micrometer and sub-micrometer level has been overcome with the advent of SKM format which offers a new and unique tool to image the electrical potential on surfaces at the micrometer and sub-micrometer level. It has also been possible to develop an SKM instrument that is capable of generating both CPD and surface topographical images in tandem1. Such equipment not only provides an electrical image of a surface, but also generates a truly tandem topographical image. Accordingly, electrical information can be integrated fully with chemical and morphological details, an extremely valuable feature for the users of the surface characterization technologies.
However, to measure the CPD on a small scale with high precision it is necessary to control closely the distance between the tip and the sample. This has been initially achieved by processing the harmonics of the Kelvin current. However, this approach leads to instability and is unreliable.